1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission of a vehicle, the control apparatus having a plurality of solenoid valves controlled by an electronic control circuit to thereby perform speed changing of a plurality of speed transmission trains of a forward running range.
2. Description of Related Art
When a control apparatus for an automatic transmission of a vehicle is constituted into an electronically controlled type using solenoid valves, it becomes necessary to enable the vehicle to run at a predetermined transmission train even at the time of a system failure in order to secure a fail-safe function when the electric charging to all of the solenoid valves is stopped in a system failure due to damages to the wires or the like.
If the above-described predetermined transmission train is a low-speed transmission train, when the system failure has occurred while running at a high speed, a shock will occur by downshifting from a high-speed transmission train to a low-speed transmission train. As a solution, the transmission train to be established at the time of system failure is set to be a relatively high speed. However, since it becomes difficult for the vehicle to start only with the high-speed transmission train, it is desirable to enable to establish also a relatively low-speed transmission train.
There has hitherto been known the following in Japanese Published Unexamined Patent Application No. 88322/1995. Namely, in a control apparatus for an automatic transmission in which an automatic transmission range and a manual transmission range can be selected as a forward running range, there is provided a changeover valve which can selectively establish two speed transmission trains at the time of a system failure. In the automatic transmission range, the changeover valve is restricted by a line pressure to the position in which, between the above-described two speed transmission trains, a first speed transmission train on a high speed side can be established. When the manual transmission range is selected, the restriction of the changeover valve by the line pressure of the changeover valve is released by a manual valve which moves in an interlocked relationship with the range switching operation so that the changeover valve is switched to a position in which, between the above-described two speed transmission trains, a second speed transmission train on a low speed side can be established at the time of system failure. In this manner, at the time of system failure, in the automatic transmission range, the first speed transmission train is established and, in the manual transmission range, the second speed transmission train is established.
In the above-described conventional control apparatus, there is a disadvantage in that, when the system failure has occurred in the manual transmission range, shocks occur due to the downshifting to the second speed transmission train of a lower speed even when the vehicle is running at a high speed. Further, there is a necessity of switching the position of the manual valve between the automatic transmission range and the manual transmission range. As a result, there is a disadvantage in that the stroke of the manual valve becomes large.
In view of the above points, the present invention has an object of providing a control apparatus for an automatic transmission of a vehicle in which, even in case an automatic transmission range and a manual transmission range is capable of being selected as a forward running range, two speed transmission trains of a high speed and a low speed can be established at the time of a system failure in the forward running range without switching the position of the manual valve. Also, the downshifting to a low speed transmission train due to the system failure while running at a high speed can be prevented.
In order to attain the above and other objects, the present invention is a control apparatus for an automatic transmission of a vehicle, the control apparatus comprising: a plurality of solenoid valves controlled by an electronic control circuit to thereby perform speed changing of a plurality of speed transmission trains in a forward running range; and switching means for selectively establishing predetermined two speed transmission trains out of the plurality of speed transmission trains in the forward running range, at a time of system failure in which electric charging to all of the solenoid valves is stopped; wherein the switching means is constituted such that: at the time of the system failure while running in the forward running range, a first speed transmission train on a high speed side out of the two speed transmission trains is established; and that, when the switching means is switched to the forward running range after switching to a range which is incapable of being switched directly from the forward running range, a second speed transmission train on a low speed side out of the two speed transmission trains is established.
The present invention is also applicable to a control apparatus in which the automatic transmission range and the manual transmission range can be selected as the forward running range. When the system failure has occurred while running in the forward running range, a first speed transmission train of a high speed is established in both the automatic transmission range and in the manual transmission range. It is thus possible to prevent the occurrence of shocks by the downshifting to the second speed transmission train of a low speed. In addition, when the system has failed, if a switching is made to the forward running range after switching to a range that cannot be directly switched from the forward running range, the second speed transmission train of a low speed is established. Therefore, in order to establish the second speed transmission train, there is no need of switching the position of the manual valve between the automatic transmission range and the manual transmission range. Further, also when only the automatic transmission range is set as the forward running range, the first speed transmission train and the second speed transmission train can be established at the time of system failure.
The range selecting pattern of the automatic transmission is such that the parking range, the reverse running range, the neutral range and the forward-running range are arranged in series in the order mentioned. The range that cannot be directly switched from the forward running range corresponds to the reverse running range and the parking range. Here, it is considered to arrange such that, at the time of the system failure, the second speed transmission train is established when a switching has been made to the forward running range after switching to the neutral range. However, since the neutral range is a range that can be directly switched from the forward running range, there is a possibility that the range is switched to the neutral range while the vehicle is running. In addition, in such an arrangement, when the system failure has occurred, the second speed transmission train is established by temporarily changing to the neutral range while running and then returning to the forward running range. Therefore, there cannot be eliminated the possibility of the second speed transmission train""s being established while running at a high speed. On the other hand, according to the present invention, at the time of the system failure, the second speed transmission train cannot be established unless the vehicle is once stopped and the range is intentionally switched to the reverse running range or to the parking range and then is returned back to the forward running range. Therefore, there can be surely prevented the possibility that the second speed transmission train is established while the vehicle is running at a high speed.